In the semiconductor industry, critical steps in the production of integrated circuits are the selective formation and removal of films on an underlying substrate. Typical processing steps involve (1) depositing a film; (2) patterning areas of the film using lithography and etching; (3) depositing a film which fills the etched areas; and (4) planarizing the structure by etching or chemical-mechanical polishing (CMP).
In film removal processes, it is extremely important to stop the process when the correct film thickness has been removed (that is, when the endpoint has been reached). In a typical CMP process, a film is selectively removed from a semiconductor wafer by moving the wafer, in a rotating or linear motion, against a polishing pad (or moving the pad against the wafer, or both) with a controlled amount of pressure in the presence of a slurry. Overpolishing (removing too much) of a film renders the wafer unusable for further processing, thereby resulting in yield loss. Underpolishing (removing too little) of the film requires that the CMP process be repeated, which is tedious and costly.
A number of methods have been suggested for obtaining reliable endpoint detection in CMP processing. These involve the following types of measurement: (1) timing the process; (2) friction or motor current; (3) capacitive; (4) optical; (5) acoustical; and (6) inductive.
In addition, U.S. Pat. No. 5,399,234 to Yu et al. describes monitoring the CMP endpoint by sending acoustic waves through a slurry containing potassium hydroxide. A chemical reaction between the potassium hydroxide and the layer being polished yields a reaction product whose concentration decreases as the endpoint is reached; this decrease is associated with a change in the acoustic velocity.
These endpoint detection methods each have inherent disadvantages, such as a lack of sensitivity, an inability to provide real-time monitoring, or requiring removal of the wafer from the process apparatus to test for endpoint.
U.S. Pat. No. 5,559,428 to Li et al. describes an in-situ endpoint detection scheme for conductive films, using an induction method. There remains a need for an in-situ, real-time endpoint detection scheme suitable for use with non-conductive films. Such a scheme should also have high detection sensitivity and fast response time (preferably less than 1 second). In addition, it is desirable that the detection apparatus be robust, inexpensive and require little maintenance.
A particularly crucial application of endpoint detection in CMP processing involves removal of a silicon dioxide (SiO.sub.2) film overlying a silicon nitride (Si.sub.3 N.sub.4) film. FIG. 1A shows a typical CMP apparatus 10 in which a workpiece 100 (such as a silicon wafer) is held face down by a wafer carrier 11 and polished using a polishing pad 12 located on a polishing table 13. FIG. 1B is a detail view showing a thin layer 102 of nitride with an overlying layer 104 of oxide. Generally, it is necessary to remove the target film of oxide so as to completely expose the stopping film of nitride, while leaving the stopping film essentially intact (see FIG. 1C). An additional requirement is that, when the process endpoint is reached, the distance between the silicon/nitride interface 107 and the surface 105 of the oxide be controlled within .+-.200 .ANG. of the target distance. This is referred to in the art as a target window of .+-.200 .ANG.. Since the oxide removal rate is typically 50 .ANG./sec, the process time must be controlled to within .+-.4 sec. Accordingly, a successful endpoint detection scheme must detect exposure of the nitride layer with very high sensitivity, and automatically stop the CMP process within a few seconds after the nitride becomes exposed (that is, no operator intervention should be required when endpoint is reached). Furthermore, the endpoint detection scheme should be effective regardless of the pattern factor of the wafer (that is, even if the area of the exposed underlying layer is a small portion of the total wafer area).